Late Quaternary Extinction of a Tree Species in Eastern North America
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Late Quaternary extinction of a tree species in eastern North America Stephen T. Jackson* and Chengyu Weng† Department of Botany, University of Wyoming, Laramie, WY 82071 Edited by Margaret Bryan Davis, University of Minnesota, St. Paul, MN, and approved September 27, 1999 (received for review July 8, 1999) Widespread species- and genus-level extinctions of mammals in been dated at 14 sites between 25,250 and 12,430 yr B.P. (16, 17). North America and Europe occurred during the last deglaciation Megafossils of temperate taxa (Quercus spp., Juglans nigra, Acer [16,000–9,000 yr B.P. (by 14C)], a period of rapid and often abrupt sp., Carpinus caroliniana, Fagus grandifolia, Carya spp., Ulmus climatic and vegetational change. These extinctions are variously americana, and Juniperus virginiana) also occur in the deposits. ascribed to environmental change and overkill by human hunters. Pollen assemblages dating 24,670 to 17,530 yr B.P. indicate that By contrast, plant extinctions since the Middle Pleistocene are regional uplands were dominated by Picea, with small popula- undocumented, suggesting that plant species have been able to tions of Quercus and other hardwoods (17). The pollen assem- respond to environmental changes of the past several glacial͞ blages lack modern analogs in eastern North America (18). interglacial cycles by migration. We provide evidence from mor- Ovulate cones from the deposits were originally identified as phological studies of fossil cones and anatomical studies of fossil the boreal species Picea glauca (16, 19), although this identifi- needles that a now-extinct species of spruce (Picea critchfieldii sp. cation was questioned by some observers (20, 21). The cones nov.) was widespread in eastern North America during the Last have been variously ascribed to an extinct species (21), or an Glacial Maximum. P. critchfieldii was dominant in vegetation of the extinct variety (22) or ecotype (23) of P. glauca. We examined 45 Lower Mississippi Valley, and extended at least as far east as cones or cone fragments from eight Tunica Hills exposures western Georgia. P. critchfieldii disappeared during the last degla- (25,250 to 16,645 yr B.P.). Cone, scale, bract, and seed dimen- ciation, and its extinction is not directly attributable to human sions are larger than modern P. glauca, and are on different activities. Similarly widespread plant species may be at risk of allometric trajectories (Figs. 2–4). Morphometric differences ECOLOGY extinction in the face of future climate change. between cones of P. glauca and the fossil cones are at least as great as those between most pairs of extant North American idespread Late Quaternary species- and genus-level ex- Picea species (Table 1). Wtinctions of mammals in North America and Europe have The fossil cones cannot be assigned to any of the other extant been documented since the early 19th Century (1–4). These North American Picea species; they differ in shape, size, phyl- extinctions were concentrated during the last deglaciation lotaxy, scale size and shape, and bract size (24–30) (Table 1). [16,000 to 9,000 yr B.P. (by 14C)] (5, 6), a period of rapid and Neither are they assignable to the late Tertiary species Picea often abrupt climatic and vegetational change (7–9). In contrast banksii (31), which differs in cone shape, phyllotaxy, and bract to mammals, few Quaternary plant extinctions are documented apex-shape. We also examined 50 foliage needles and needle (10–14). Most are concentrated at the Pliocene͞Pleistocene fragments associated with the cones from six exposures (25,250 boundary (ca. 1.6 million yr ago), although a few occurred in to 17,530 yr B.P.). None were assignable to any of the extant Europe during the Early to Middle Pleistocene. No Late Qua- North American species, differing in apex shape, angularity, and ternary plant extinctions have been reported, leaving the im- hypoderm sclerification, as well as in size, continuity, and pression that few or no extinctions occurred during the last position of resin ducts (see ref. 32; Table 1; Fig. 5). glacial͞interglacial cycle. Most knowledge of Late Quaternary Cones of P. glauca and Picea mariana occur at other sites that vegetational and floristic change comes from fossil pollen pre- are contemporaneous with the Tunica Hills deposits (33, 34), served in lake and wetland sediments. Species extinctions may be and needles of both species are documented from contempora- masked by taxonomic smoothing in the pollen record, which neous sites to the north (C.W., S.T.J., and N. G. Miller, unpub- rarely permits species-level differentiation. Plant macrofossils lished data). The occurrence in the Tunica Hills of morpholog- often support species-level identification, but few sites have been ically unique cones in association with morphologically and studied in detail, and few studies have included the detailed anatomically unique needles, and the absence of other cone or morphological and anatomical analyses required to determine needle morphotypes in the deposits indicates that they belong to Picea critchfieldii whether fossil material is unequivocally assignable to extant or a single, now-extinct species, Jackson & Weng, sp. nov. extinct species. We provide evidence from morphological studies of fossil Picea critchfieldii Jackson & Weng, sp. nov.: Specific Diagnosis. Ovu- cones and anatomical studies of fossil needles for the Late late cones cylindrical, 60(?)-100 mm long, 14–20 mm diameter. Quaternary extinction of a species of Picea (spruce) that was Scale phyllotaxy 3͞5. Cone scales narrowly fan-shaped with widespread in the southeastern United States during, and im- rounded margins, 18–21 (12–25) mm long, 11–13.5 (7–16.5) mm mediately before, the Last Glacial Maximum. The organically wide; cone scale margin entire to moderately erose. Subtending preserved fossils are not assignable to any extant Picea species of bracts on cone scales spatulate, 5.5–7 (3.0–8.3) mm long, 1.5–1.8 North America; they occur at sites ranging from the Lower Mississippi Valley to the Atlantic Coastal Plain and Piedmont; and they are contemporaneous with fossils of extant eastern This paper was submitted directly (Track II) to the PNAS office. North American Picea species. Abbreviation: B.P., time before the present, as calculated by 14C. Most of the fossils are from streamcut exposures of Late *To whom reprint requests should be addressed. E-mail: [email protected]. Quaternary fluvial deposits in the Tunica Hills upland of ͞ Ј †Current address: Department of Biological Sciences, Florida Institute of Technology, Louisiana Mississippi (31°N, 91°29 W; see Fig. 1). Silts and clays Melbourne, FL 32901. accumulated in low-energy backwater pools and abandoned The publication costs of this article were defrayed in part by page charge payment. This channels (15, 16) contain abundant Picea megafossils, including article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. ovulate cones, twigs, needles, and wood (17). Picea wood has §1734 solely to indicate this fact. PNAS ͉ November 23, 1999 ͉ vol. 96 ͉ no. 24 ͉ 13847–13852 Downloaded by guest on October 2, 2021 Fig. 1. Paleogeography of eastern North America at the time of the Last Glacial Maximum (18,000 yr B.P.). TH and BB represent Tunica Hills and Bob Black Pond, respectively, where Picea critchfieldii cones and͞or foliage occur in sediments of the Last Glacial Maximum and Farmdalian Interstadial. AC and Fig. 3. (A) Seed and seedwing of P. critchfieldii (TB-23/1093/15). (B–H) NC represent the Andersonville Claypit and Nonconnah Creek sites, respec- Representative cone scales of P. critchfieldii from Tunica Hills sediments. B–D, tively, where cones tentatively assigned to this species occur in Late Wiscon- G, and H are typical of scales from middle portions of cones. Some scales are sinan sediments. abraded or are missing small portions, and bracts are missing from some specimens. (B) PC-19/1191/8; (C) TB-23/1093/15; (D) TB-23/0393/8; (E) PC-21/ 1092/1; (F) LBS-4/1191/5; (G and H) TB-22/0293/1. (I and J) Representative cone (1.3–2.3) mm wide; bract apex rounded and minutely toothed. scales of modern P. glauca (I) Franklin Co., New York; (J) Cypress Hills, Seeds ovate, 3.5–4.5 (2.5–5.2) mm long, 2.6–2.8 (2.0–3.4) mm Saskatchewan). (K and L) Representative cone scales of modern Picea chihua- wide, each with a membranous wing 8–11 (4.5–13) mm long. huana from Chihuahua, Mexico. (M and N) Representative cone scales of Needles 7–9 (4–11) mm long, 0.6–1.0 mm diameter; quadran- modern Picea breweriana from Siskiyou Co., California. (O and P) Represen- tative cone scales of modern Picea martinezii from Nuevo Le´on, Mexico. gular in cross-section; apex acute; two resin ducts spanning entire needle length; resin ducts 60–80 (40–100) m in diameter, positioned in needle angles. Holotype: University of Wyoming Quaternary Plant Ecology Laboratory collections, TB-23/1093/ 15. Paratypes: PC-19/1191/1, PC-19/1191/8, TB-23/0393/8, TB- 22/0293/1, PC-21/1092/1, PC-21/1092/2, PC-21/1092/4, LBS-1/ 0592/6, LBS-4/1191/5. Etymology: Named in honor of the late William B. Critchfield in recognition of his advocacy of under- standing the role of Quaternary history in shaping genetic structure of conifer populations. He encouraged the senior author to investigate Quaternary spruce paleobotany and hence was partly responsible for initiating this study. Discussion. P. critchfieldii was not restricted to the Tunica Hills region (Fig. 1). Cones of similar dimension and morphology have been reported from sediments dating to 17,195 yr B.P. in western Tennessee (23), and 21,300 yr B.P.